Prime Minister David Cameron today said there was a "limit" to the amount of money that he will spend on the HS2 rail link between London and the North, but insisted: "HS2 is going to happen."

The future of the proposed line linking the capital with cities including Birmingham, Manchester and Leeds was thrown into doubt at last week's Labour conference, when shadow chancellor Ed Balls said he would not provide a "blank cheque" for the project and would have to consider whether there were better uses for £50 billion of state spending.

But Mr Cameron insisted that the massive construction project can be built within the Government's budget of £42.6 billion and rejected suggestions that other more useful transport schemes were being crowded out.

Asked if the Government was ready to spend unlimited amounts on HS2, Mr Cameron told BBC1's Andrew Marr Show: "Of course there's a limit. We are not going to just spend any money on this. We have a budget - £42.6 billion.

Still image from the HS2 promotional video

"This is the Government that delivered the Olympics on time and on budget. We are delivering Crossrail - the biggest construction project anywhere in Europe - on time and on budget. HS2 is going to happen. "

Mr Cameron said that the main argument for HS2 was the need for more capacity on the rails.

"The west coast main line is full," he said. "We have to build another one. The choice is do we build an old-style Victorian one or do we build one of the new high-speed lines?

"I think people all over the country are worried that HS2 is going to take up too much money.

"In the next Parliament, we will be spending more than three times as much on other transport projects as we will on HS2.